1. Field of the Invention
The present invention relates to an injector that injects and supplies fuel to an engine.
2. Description of Related Art
Conventionally, an injector is mounted on a direct injection engine (e.g., a diesel engine), which receives fuel from a fuel supply source (e.g., common rail) for directly injecting and supplying the fuel into a cylinder of an engine.
Recently, in order to improve an efficiency of combustion by further atomizing fuel spay of the fuel injected through the injector, injection pressure of the fuel by the injector has been increased. And, there has been a study for even aggressively increasing the pressure by providing a boosting mechanism (intensifier mechanism) to the injector, not only by increasing supply pressure of the fuel in a fuel supply source.
For example, the boosting mechanism includes a boosting piston (intensifier piston), which integrally includes a large-diameter piston member and a small-diameter piston member. Here, the large-diameter piston member is slidably received in a large-diameter cylinder, and the small-diameter piston member is slidably received in a small-diameter cylinder. Also, the boosting mechanism blocks the large-diameter cylinder by the large-diameter piston member to form (define) a boosting chamber, which the fuel as a boosting medium flows into and out of. Also, the boosting mechanism blocks the small-diameter cylinder by the small-diameter piston member to form (define) a boosted chamber, which fuel to be boosted flows into and out of.
Then, the boosting mechanism boosts the fuel (increases the pressure of the fuel) in the boosted chamber in accordance with an area ratio between an end face (boosting surface) of the large-diameter piston member and an end face (boosted surface) of the small-diameter piston member. Here, the end face (boosting surface) is exposed to the boosting chamber and applies pressure to the fuel in the boosting chamber, and the end face (boosted surface) is exposed to the boosted chamber and applies pressure to the fuel in the boosted chamber (see, for example, JP-A-2003-106235).
By the way, in order to retain oil tightness in the boosting mechanism, a clearance (large-diameter side slide clearance) and a clearance (small-diameter side slide clearance) both need to be set small to be, for example, 1˜5 m. Here, the clearance (large-diameter side slide clearance) is a clearance between the inner peripheral surface of the large-diameter cylinder and the outer peripheral surface of the large-diameter piston member, and the clearance (small-diameter side slide clearance) is a clearance between the inner peripheral surface of the small-diameter cylinder and the outer peripheral surface of the small-diameter piston member.
However, when the boosting piston is formed in a condition, where the large-diameter piston member and the small-diameter piston member are integrally formed with each other and are generally coaxially to each other, an erroneous measurement of an axial center position of the large-diameter piston member against that of the small-diameter piston member needs to be set smaller than a total measurement of the large-diameter and small-diameter side slide clearances in order to retain appropriate slidability. Then, it is very difficult to machine the boosting piston with a high degree of accuracy in the coaxiality between the large-diameter piston member and the small-diameter piston member under a condition where the large-diameter and small-diameter side slide clearances are set smaller as above.
Further, when the injector is assembled and fixed to the engine, the injector is applied with a very large fixing force. Therefore, because of the fixing force, the error may be generated in the axial center positions between the large-diameter piston member and the small-diameter piston member. Then, in the boosting piston, where the large-diameter and small-diameter side slide clearances are set small and also the coaxiality is formed highly accurately, slide deficiency of the large-diameter and small-diameter piston members may occur with a very high probability due to the above error of the axial center positions.
Thus, according to the conventional boosting mechanism, in order to retain the oil tightness in the boosting mechanism, the large-diameter and small-diameter side slide clearances need to be set small. But on the other side, the slide deficiency of the large-diameter and small-diameter piston members may occur with the very high probability due to the fact that the large-diameter and small-diameter side slide clearances are set small. Therefore, the injector having the conventional boosting mechanism has difficulty in retaining both the oil tightness and the slidability.